1. Field of the Invention
The present invention relates to a super-twisted nematic (to be referred to as an STN hereinafter) simple matrix type liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices are widely used as display devices for office automation equipment. Such a display device is required to realize high-definition display and hence needs to have a large number of pixels and perform high time-division driving. In addition, with regard to display characteristics, the device is required to realize high contrast and a wide viewing angle. In order to meet these requirements, an STN simple matrix type liquid crystal display device, which is capable of high time-division driving and has relatively high contrast as compared with other types of liquid crystal display devices, is used as a display device for a personal computer or the like.
This STN simple matrix type liquid crystal display device comprises a pair of substrates arranged to oppose each other through a predetermined gap, electrodes arranged on the opposing inner surfaces of the pair of substrates to cross each other at right angles, a pair of aligning films respectively covering the electrode-formed surfaces of the substrate to align liquid crystal molecules in a predetermined direction, a liquid crystal material sealed between the aligning films, and a pair of polarizing plates arranged outside the pair of substrates to sandwich these components. The molecules of the liquid crystal material sealed between the pair of aligning films, which molecules are located near the respective aligning films, are aligned in the aligning treatment direction predetermined by the alignment forces of the aligning films, so that the molecules may be twisted at an angle of about 240.degree. from one substrate to the other substrate.
When the liquid crystal display device is driven by a time-division driving scheme, and an electric field is applied between the opposing electrodes, the alignment of the liquid crystal molecules is changed. An optical change accompanying this alignment change is visualized by the pair of polarizing plates, thus performing desired display.
In the above-described liquid crystal display device, however, in order to realize high time-division driving, the twist angle of the liquid crystal molecular alignment is set to be large, while a birefringent effect is used to achieve high visual contrast. For this reason, coloration of display contents is caused. In addition, the viewing angle is not sufficiently wide, and a change in display color occurs depending on a visual angle.
In order to solve these problems, a two-layered STN liquid crystal display device has been proposed, in which a drive cell having drive electrodes formed on the above-mentioned pair of substrates and a compensation cell having liquid crystal molecules twisted in a direction opposite to the twist direction of the liquid crystal molecules of the drive cell are stacked to form a two-layered structure. In this two-layered STN liquid crystal display device, differences in phase difference caused by light components having different wavelengths in the drive cell can be corrected by the compensation cell to suppress coloration of the display contents, thus allowing display close to monochrome display.
In such a two-layered STN liquid crystal display device, since a liquid crystal cell similar to the drive cell is used as the compensation cell, the manufacturing process of the compensation cell is complicated, resulting in an increase in cost and size of the display device. In addition, coloration of the display contents is not sufficiently eliminated, and the conventional problems, i.e., an insufficient viewing angle and changes in display color with visual angle are left unsolved.